1. Field of the Invention
The present invention relates to a device for dispersing gas, e.g., air, into a liquid or a slurry which is in motion. As a specific example of one field of application of the invention flotation cells may be mentioned.
In addition to carrying out flotation, the invention can be applied to any process in which air is caused to mix as fine bubbles into a liquid or slurry.
2. Description of the Prior Art
Froth flotation is used in mineral processing to separate valuable minerals from gangue. Generally speaking, when the flotation process is used, different types of particles are separated from each other at the interface of two flowing phases which are in contact; at least one of the phases must be a liquid. The interface can be formed by two liquids or a liquid and a gas. The separation can take place, for example, by causing part of the particles to adhere to the said interface by means of suitable chemicals while the others do not.
Devices called flotation machines, flotation cells, or flotators are used in the practical application of the flotation process. The task of the flotation machine is:
1. To produce the said interface between, for example, a liquid phase and a gas phase.
2. To bring the particles to be separated and the said interface into contact with each other.
3. To transfer the particles to be recovered and those to be rejected in their own directions.
It is advantageous for the efficiency of the separation in the flotation process that the interface between the phases is as large as possible and that the impact of the particles against the interface is as large as possible. In a modification, the same applies if the question is of the dissolving of a solid or a liquid into another liquid.
Especially regarding the froth flotation phenomenon it must be noted that the amount of energy consumed by the flotation machine alone does not guarantee a good technological and economic result, but the decisive factor is, following the above principles, that the dispersion of the gas is efficient, that the mineral particles remain in motion, and that the formation of gas-bubble mineral-particle aggregates is effective.
The number of flotation machine models has been immense in the 60 years during which the froth flotation technique has been used to a noteworthy extent. Most of the models have been created only to disappear. The principal types currently on the market are as follows:
1. Pneumatic flotation machines in which the ore slurry is kept as a suspension by blowing low-pressure air to the bottom of the machine through pipes, nozzles, etc. The proportion of machines of this type is, however, relatively small at present.
2. Mechanical flotation machines in which a rotor which constitutes a part of the driving mechanism of the machine sucks the air from outside the machine and disperses it into the slurry and keeps the slurry as a suspension.
3. Mechanical-pneumatic flotation machines in which the role of the rotor is to keep the slurry as a suspension but in which the requisite air is fed by overpressure from outside the machine. The air is usually dispersed by the rotor.
In the mechanical and mechanical-pneumatic flotation machines the driving mechanism consists of a rotor-stator combination. At present the most common types of mechanisms on the market are:
1. The so-called Denver mechanism the prototype of which was patented for the first time by A. W. Fahrenwald in 1934 (U.S. Pat. No. 1,984,366). It has a rotating horizontal disk-like impeller with low ribs on both the upper and the lower surface. Around the impeller there is a stabilizer provided with fixed radial ribs on the lower side. The impeller takes air through a stand pipe around the shaft of the machine. The sides of the ribs which face each other in the impeller and the stabilizer are vertical.
2. The so-called Fagergren mechanism (U.S. Pat. No. 1,963,122 dated 1934). In its older form both the rotor and the stator are similar "cages" assembled from two end rings and vertical tubes between them.
The rotating rotor inside the stationary stator sucks the slurry with its lower end impeller and air through the stand pipe surrounding the shaft with its upper end impeller.
In the newer Fagergren type the previously some 300-400 mm long tubes have been converted into vertical blades which start directly from the rotor center and widen towards the periphery, while the stator is a cylinder which surrounds the rotor and has a perforated mantle.
3. In the Agitair flotation machine (U.S. Pat. No. 3,327,851) the rotor consists of a rotating plate with a smooth upper surface which has downwards-directed, vertical "fingers" at regular intervals along its edges. The stator consists of radial, rectangular plates; the rotor rotates in its open center.
4. The horizontal, disk-like rotor of the driving mechanism of the VK-3 flotation machine (Finnish Pat. No. 45,416) has blades on both sides. Above the rotor there is a closed stop plate; when the rotor rotates, a strong suction is produced in the space between the plate and the rotor surface to admit slurry into the machine. The low pressure air fed through the hollow shaft is dispersed into the slurry by the lower blades of the rotor when it flings the slurry through the spaces between the plates of the stator surrounding the rotor and assembled from radial, rectangular, vertical plates.
The flotation machines on the market in addition to those described above are more or less variations of the same. They all seem to have in common the characteristic that the machine construction is based on practical experience and momentary revelation rather than understanding the underlying physical phenomena.